Hey there folks, not sure if this problem is solely related to source games but any time I start Left 4 Dead 2 or No More Room In Hell my headphones would quickly turn off completely.
That is if I start/being a campaign level for either one, only way I was able to "Fix" the problem for L4D2 was by changing the audio type from "headphones" to "speakers" and back to "headphones".
I have tried that very same tactic on NMRIH but I had no audio come up, note that I have a wireless headphone and maybe that's a reason why the audio cuts off but I'm not sure. Also I don't have any speakers of any kind which is why I rely on my headset to be able hear any form of danger, won't be able to tell If I got grabbed by a crawler without that alert!